As genetically encoded reporter molecules, fluorescent proteins have demonstrated applicability and versatility as molecular and cellular probes in life sciences and biomedical research. Among patents relating to fluorescent protein technology are U.S. Pat. Nos. 5,491,084, 5,625,048, 5,777,079, 5,804,387, 5,968,738, 5,994,077, 6,027,881, 6,054,321, 6,066,476, 6,077,707, 6,124,128, 6,090,919, 6,172,188, 6,146,826, 6,969,597, 7,150,979, 7,157,565, 7,166,444, 7,183,399 and 7,297,782, references incorporated herein.
Fluorescent protein fusion to a gene promoter has been employed for reporting or verifying gene expression. Fluorescent protein fusion to a gene of interest has also been used to track a protein as it traverses a cell. If the fusion partner is a structural protein, then information pertaining to cellular architecture may be obtained. Fluorescent proteins have found application in a vast array of experiments, included those relating to monitoring gene promoter activity, gene expression levels, organelle dynamics, cellular architecture, gene expression timing, protein translocation, G-protein-coupled receptor (GPCR) activity, cell lineage, apoptosis, protein degradation, genotoxicity and cytotoxicity.
Cell-based assays are increasingly gaining in popularity in the pharmaceutical industry due to their high physiological relevance. Additional advantages include their ability to predict compound usefulness, evaluate molecular interactions, identify toxicity, distinguish cell type-specific drug effects, and determine drug penetration. Cell-based assays are relevant throughout the drug discovery pipeline, as they are capable of providing data from target characterization and validation to lead identification (primary and secondary screening) to terminal stages of toxicology. Current industry trends of performing drug screening with cell context demand easily monitored, non-invasive reporters. Fluorescent proteins fulfill this demand more completely than any other available tools. Requirements for advanced screening assays are driven by the objective of failing candidate compounds early in the drug discovery pipeline. This fundamental approach increases efficiency, reduces costs, and results in shorter time to market for new drugs. In order to fail compounds early, information-rich data for accurate early-stage decision making is required. Such data may be derived by screening compounds in context, that is, by screening in relevant living systems, rather than with classical biochemical assays, often incorporating sophisticated imaging platforms, such as high-content screening (HCS) workstations. The industrialization of fluorescent microscopy has led to the development of these high-throughput imaging platforms capable of HCS. When coupled with fluorescent protein reporter technology, HCS has provided information-rich drug screens, as well as access to novel types of drug targets.
As industry trends advance toward analysis in living systems (e.g. cells, tissues, and whole organisms), fluorescent proteins, by virtue of their non-invasive, non-destructive properties, are becoming indispensable tools for live-cell analysis. A broad range of fluorescent protein genetic variants are now available, with fluorescence emission profiles spanning nearly the entire visible light spectrum. Mutagenesis efforts in the original jellyfish Aequorea victoria green fluorescent protein have resulted in new fluorescent probes that range in color from blue to yellow and these are some of the most widely used in vivo reporter molecules in biological research today. Longer wavelength fluorescent proteins, emitting in the orange and red spectral regions, have been developed from the marine anemone Discosoma striata and reef corals belonging to the class Anthozoa. Other species have also been mined to produce similar proteins having cyan, green, yellow, orange, red, and even far-red fluorescence emission.
Recent emphasis on multi-color imaging in HCS has created renewed demand for easily measured, non-invasive, and non-disruptive cellular and molecular probes. With the increasingly expanding repertoire of fluorescent proteins has come increased demand for complementary reagents, such as organic fluorochrome counter-stains that augment analysis by providing information relating to co-localization of the fluorescent proteins to various organelles and subcellular targets. To date, however, concerted efforts in developing such organic fluorochromes, specifically tailored to working in concert with fluorescent proteins, has been limited in scope. The application of fluorescent proteins and of organic fluorochromes is not an either/or proposition. Each technology has distinct advantages and limitations. These two technologies can be optimized and combined to work in concert, however, in order to maximize the information content obtained from fluorescence microscopy- and imaging-based screening approaches. By doing so, achieving rich multi-dimensional physiological information can be obtained.
While suitable for analysis of cell surfaces and permeabilized cells, fluorescently-labeled antibodies have few practical applications for intracellular imaging in living cells, due to their inherent inability to penetrate to their targets, which has given rise to development of cell-permeable small molecule organic fluorochromes, certain ones of which naturally sequester inside-specific organelles, based upon biophysical or biochemical properties favoring that distribution. Acceptable small molecule organic probes for cell imaging and analysis need to be minimally perturbing, versatile, stable, easy-to-use, and easy to detect using non-invasive imaging equipment. A problem with the classical organic probes from histology is that many of them require cofactors or, by requiring fixation or staining, report only on the static condition of a dead cell. The required additional steps may be time consuming and expensive and, in the case of fixing and staining, may lack biological relevance. In the context of the analyses described above, an organic probe must be able to report upon events in living cells and in real time. Simplicity is of key importance, especially in the context of drug screening.
While various organic fluorochromes have been developed in the past for live cell analysis, typically they were not devised with optimization of performance in conjunction with the wide palette of available fluorescent proteins in mind. For instance, several U.S. patent documents (U.S. Pat. Nos. 5,338,854, 5,459,268, 5,686,261, 5,869,689, 6,004,536, 6,140,500 and 6,291,203 B1, as well as US Patent Applications 2005/0054006 and 2007/0111251 A1, references incorporated herein) disclose organic fluorochromes which are described as useful for visualizing membranes, mitochondria, nuclei and/or acidic organelles. Additional examples of various fluorochromes and their application in biological imaging may be found in the published literature (see, for example, Pagano et al, 1989; Pagano et al, 1991; Deng et al, 1995; Poot et al, 1996; Diwu et al, 1999; Rutledge et al, 2000; Lee et al, 2003; Bassøe et al, 2003; Rosania et al, 2003, Li et al 2007; Boldyrev et al, 2007; Nadrigny et al, 2007). These dyes have been created using a number of fluorophores, most commonly dipyrrometheneboron difluoride (BODIPY), cyanine, carbocyanine, styryl and diaminoxanthene core structures. Typical emission maxima for these organic fluorophores span from 430 to 620 nm. Many of the dyes consequently occupy valuable regions of the visible emission spectrum that preclude use of various fluorescent proteins. By doing so, their use limits the overall levels of multiplexing achievable in HCS assays. Additionally, these dyes often display other suboptimal properties, such as a propensity to photo-bleach, metachromasy and even a tendency to photo-convert to different emission maxima upon brief exposure to broad-band illumination.
Artifacts Associated with Previously Devised Organic Fluorochromes for Live Cell Analysis
Fluorescence co-localization imaging is a powerful method for exploring the targeting of molecules to intracellular compartments and for screening of their associations and interactions. In these kinds of experiments, distinct fluorochromes and/or fluorescent proteins of interest are imaged as spectrally separated detection channels. The fluorescence intensity in each channel is ideally dominated by spatial and concentration information derived from one fluorophore only. Many commercially available organic fluorophores for subcellular analysis are disadvantaged in displaying suboptimal properties relating to these types of applications.
Lysotracker Red DND-99 (Invitrogen, Carlsbad, Calif.) contains a BODIPY fluorophore in the form of a conjugated multi-pyrrole ring structure and also contains a weakly basic amine that causes the fluorochrome to selectively accumulate in acidic compartments, exhibiting red fluorescence upon appropriate illumination (excitation: 577 nm, emission: 590 nm) (Freundt et al, 2007). Lysotracker Red is structurally related to Lysotracker Green but the former has an additional pyrrole ring in conjugation with the primary structure, which produces a longer wavelength emission. Lysotracker Red has commonly been used in multi-color imaging studies as a lysosomal marker to determine intracellular localization of GFP-tagged proteins by fluorescence or confocal microscopy. Excitation of the red-emitting molecule with broad-band illumination induces, however, molecular changes rendering its photochemical properties similar to those of Lysotracker Green. The similarities between the spectra of Lysotracker Green and converted Lysotracker Red suggest that the third pyrrole ring is taken out of conjugation during the photo-conversion process, leading to a shorter wavelength dye emission. Thus, Lysotracker Red staining for epifluorescence or confocal microscopy, in conjunction with visualization of GFP, leads to spurious results due to photo-conversion of the fluorophore (Freundt et al, 2007).
Acridine orange (Sigma-Aldrich, Saint Louis, Mo. and other sources) has also been used extensively as a fluorescent probe of lysosomes and other acidic subcellular compartments. Acridine orange's metachromasy results, however, in the concomitant emission of green and red fluorescence from stained cells and tissue (Nadrigny et al, 2007). Evanescent-field imaging with spectral fluorescence detection, as well as fluorescence lifetime imaging microscopy demonstrate that green fluorescent acridine orange monomers inevitably coexist with red fluorescing acridine orange dimers in labeled cells. The green monomer emission spectrally overlaps with that of GFP and produces a false apparent co-localization on dual-color images. Due to its complicated photochemistry and interaction with cellular constituents, acridine orange is a particularly problematic label for multi-color fluorescence imaging-both for dual-band and spectral detection. Extreme caution is required, therefore, when deriving quantitative co-localization information from images of GFP-tagged proteins in cells co-labeled with acridine orange.
In principle, the styryl dye, FM4-64 (Invitrogen, Carlsbad, Calif.) is useful for studying endocytosis and vesicular recycling because it is reputed to be confined to the luminal layer of endocytic vesicles. This particular dye distributes throughout intracellular membranes and it indiscriminately stains both the endoplasmic reticulum and nuclear envelope (Zal et al, 2006). However, though the different pools of dye all emit at roughly 700 nm, a spectral shift in fluorescence excitation maximum is observed wherein the dye present in endocytic vesicles and the endoplasmic reticulum absorbs at 510 nm, while the dye associated with the nuclear matrix absorbs at 622 nm. While this can be used advantageously in order to selectively image the nuclear membrane, in certain multi-parametric imaging experiments the dual absorption properties can be problematic. The shift in peak of the absorption spectrum is not confined to FM dyes. A similar phenomenon has also been reported for Rhodamine 6G, where the dye's absorbance maximum is red-shifted from 527 to 546 nm in a concentration dependent manner (Johnson et al, 1978). Rhodamine 6G is commonly employed to label leukocytes, especially in vascular injury models.
Fluorescent analogs of ceramide are commonly employed to visualize golgi bodies in live cells. The fluorescence emission maximum of certain BODIPY-labeled ceramides, such as C5-DMD-Ceramide (a.k.a. C5-BODIPY-Cer, Invitrogen, Carlsbad, Calif.), has been shown to depend strongly upon the molar density of the probe in the membrane, shifting in emission maximum from green (˜515 nm) to red (˜620 nm) with increasing concentration (Pagano et al, 1991). Consequently, in live cells, the Golgi bodies display yellow/orange fluorescence emission (a combination of red and green fluorescence emission), whereas predominantly green fluorescence emission is observed in the endoplasmic reticuli, the nuclear envelope and mitochondria. Co-localization studies with GFP are compromised, therefore, when employing these fluorescent ceramide analogs, due to their inherent dual emission characteristics.
Only in the specific instance of nuclear staining have the aforementioned problems been alleviated to a large extent. DRAQ5™ ([1,5-Bis[[2-(dimethylamino)ethyl]amino]4,8-dihydroxyanthracene-9,10-dione], Biostatus Limited, UK) is a cell-permeable substituted anthraquinone dye designed for use in a range of fluorescence detection technologies, for the discrimination of nucleated cells (U.S. Pat. Nos. 6,468,753 B1 and 7,060,427 B2, Smith et al, 1999; 2000). The dye permits nuclear discrimination and functional assays to be performed in live cells in combination with a variety of UV and visible range fluorochromes, such as fluorescein, R-phycoerythrin and the GFP super-family. Additionally, the dye has little propensity to photo-bleach.